Fix contrail-collector back pressure mechanism

contrail-collector DB queue back presssure mechanism was not
working since the DB drop level is initialized to INVALID and
even the water marks levels are INVALID and hence the defer/undefer
callbacks are not called.

Change-Id: Ib28141a69aeed3c4ad6f50abbaed2a285e3e7db2
Partial-Bug: #1528380
(cherry picked from commit 7e4ad73)

src/analytics/db_handler.cc

@@ -147,9 +147,10 @@ void DbHandler::SetDropLevel(size_t queue_count, SandeshLevel::type level,
             Sandesh::LevelToString(level) << "], DB QUEUE COUNT: " << 
             queue_count);
         drop_level_ = level;
-        if (!cb.empty()) {
-            cb();
-        }
+    }
+    // Always invoke the callback
+    if (!cb.empty()) {
+        cb();
     }
 }